The discussions of the equation of transfer and the solution of this equation in Chapter 2 rest entirely on concepts of classical physics. Such treatment was possible because we considered a large number of photons interacting with a volume element that, although it was assumed to be small, was still of sufficient size to contain a large number of individual molecules. But with the assumption of many photons acting on many molecules we have only postponed the need to introduce quantum theory. Single photons do interact with individual atoms and molecules. The optical depth, τ(υ), depends on the absorption coefficients of the matter present, which must fully reflect quantum mechanical concepts. The role of quantum physics in the derivation of the Planck function has already been discussed in Section 1.7. Both the optical depth and the Planck function appear in the radiative transfer equation (2.1.47).

The interaction of radiation with matter can take many forms. The photoelectric effect, the Compton effect, and pair generation–annihilation are processes that occur at wavelengths shorter than those encountered in the infrared. Infrared photons can excite rotational and vibrational modes of molecules, but they are insufficiently energetic to excite electronic transitions in atoms, which occur mostly in the visible and ultraviolet. Therefore, a discussion of the interaction of infrared radiation with matter in the gaseous phase needs to consider only rotational and vibrational transitions, while in the solid phase lattice vibrations in crystals must be included.

In this chapter we review the physical foundation of remote sensing. Except for possible gravitational effects, information accessible to a distant observer must be sensed as electromagnetic radiation, either in the form of reflected or refracted solar or stellar radiation, or in the form of thermal or nonthermal emission. We restrict the discussion to passive techniques. Active methods, involving the generation of electromagnetic radiation (radar, lidar), are not explicitly treated. However, the physical principles discussed in this text are equally applicable to passive and active methods. In either case a discussion of the measurement and interpretation of remotely sensed data must be based on electromagnetic theory. In Section 1.1 we begin with that theory by reviewing Maxwell's equations. The application of the principle of energy conservation to Maxwell's equations leads to the Poynting theorem with the Poynting vector describing radiative energy transport; this is discussed in Section 1.2. However, the Poynting vector does not characterize more complex phenomena, such as reflection, refraction, polarization, or interference; all of these phenomena play significant roles in many aspects of remote sensing. Their study requires, first, a derivation of the wave equation from Maxwell's formulas, and second, finding appropriate solutions for the electric and magnetic field vectors; this is the subject of Section 1.3. Polarization is briefly reviewed in Section 1.4. Effects of electromagnetic waves striking an interface between two media and the conditions that must be satisfied at the boundary are treated in Section 1.5.

The planets had finally finished growing. Now they would begin their long process of evolution towards the way we see them today. By now, about 100 million years had passed and the Solar Nebula was relatively sparse. Yet its activity did not stop completely. For the Solar System was still littered with fragments of debris that had not yet been ejected from the system by the giants or been swept up by the terrestrials. It was at this point that the Solar System entered what astronomers call, quite justifiably, the heavy bombardment phase.

For hundreds of millions of years, leftover scraps continued to rain down on the planets and their satellites. This is the battering that shaped the planets' and moons' crusts, and the majority of it occurred in the first 600 million years or so of their creation. A glance at the surface of the Moon gives ample reminder of this violent phase in the Solar System's history. Many of the craters there are well over 100 kilometres across. One of them is about 12 kilometres deep and 2500 kilometres across – greater than half the Moon's diameter. Called the Aitken basin, it is the largest known impact structure in the entire Solar System, carved out when the Moon was struck a glancing blow from a piece of rock and metal some 200 kilometres across. This constant barrage meant that the crusts of the terrestrial planets and moons oscillated between molten and solid states for many hundreds of millions of years. The heaviest elements sank to their centres, while the lighter substances, buoyed up, stayed near the surfaces.

The Sun is already dying, in a sense. All the while our star burns hydrogen on the main sequence, its core becomes gradually more depleted in that element, and a helium ‘ash’ is left in its place. As the core adjusts itself to this steadily changing composition, the star's diameter and brightness both slowly increase. When the Sun took its first steps onto the main sequence it was only 90 per cent of its current radius and 60 or 70 per cent of its present luminosity. It is quite a bit warmer and larger now than it used to be. And that trend is not going to change.

The next billion years will see a hike in the Sun's luminosity by about 10 per cent. That may not sound like a cause for concern, but for the innermost planets the change will be overwhelming. And, for the Earth in particular, this slight escalation in luminosity will signal the beginning of the end of billions of years of evolution. With that much extra energy flowing away from the Sun, our planet's polar caps will start to melt and its oceans will begin to warm up. Slowly, they'll evaporate into the atmosphere. Too much water vapour, like carbon dioxide, has a serious effect on our planet's climate. The Sun's energy heats the surface, but the heat is partially trapped. Infrared radiation cannot travel through water vapour or carbon dioxide, because they absorb it. And so the planet steadily warms up. Today, Earth is about 32 Celsius warmer than it would be without its atmospheric blanket.

Once the collapse of the giant molecular cloud had started, it continued under its own momentum. By the time two million years had passed, a multitude of nuclei had developed in the cloud, regions where the density was higher than average. These concentrations began to pull in more gas from their surroundings by virtue of their stronger gravity, and the original cloud fragmented into hundreds or even thousands of small, dense cores. Most of them would later form stars. One of them was destined to become the Sun.

By now, the cloud core from which the Sun would form was perhaps a tenth of a light-year across, more than a hundred times the present size of the Solar System out to Pluto. Gradually, this tight clump of gas continued to fall in on itself like a slow-motion demolished chimney stack, a process known as gravitational freefall. The innermost regions fell the fastest; they were closest to the central condensation where the gravitational pull was greatest. The outermost edges of the cloud core took longer to succumb to their inevitable fall. Thus, because of these differences in infall rates, the cloud's contraction essentially amounted to an implosion, an explosion in reverse. In time, as the gas closest to the centre plunged inward and accelerated, the material there grew steadily hotter, the atoms and molecules within it rubbing against each other frantically. After perhaps millions of years in a deep freeze, the molecular cloud was finally warming up. The eventual result was a gas and dust cocoon: a shell of dark material surrounding a denser, warmer core. Such an object is known as a globule. It was the Sun's incubator.

The planets, their moons, the asteroids and the comets – all are part of the Sun's family. And they are just as ancient as their parent. Evidence suggests that the Solar System's contents started to form even while the Sun itself was still only a protostar, almost as soon as the Solar Nebula was in place.

We have seen that, in some ways, the Sun formed in much the same manner in which a sculpture is made. What began as a single, large block of material – the giant molecular cloud – was gradually whittled away to reveal a smaller end product. But the planets' origins are more like those of buildings. They grew bit by bit, from the bottom up, by accumulating steadily larger building blocks. The very first process in the planet-building production line is a familiar concept known as condensation. You can see it in action when somebody wearing spectacles enters a warm room after being outside in the cold. As soon as air-borne water molecules hit the cold lens surfaces, the molecules cool down and stick to the lenses one at a time to produce a thin – and very annoying – film of tiny water droplets. Exactly the same phenomenon was big business in the very earliest stages of the Solar Nebula. As more and more material spiralled from the Solar Nebula into the newly forming Sun, the disc grew less dense.

It is now more than four and a half billion years since the Solar System came into being. Generally there has been little evolution in the grand scheme of things. Along with some of the planets' moons, the asteroids have surfaces that have not been modified extensively since the heavy bombardment stopped, 3300 million years ago. They still populate the belt between Mars and Jupiter. The comets still surround the Sun in the Oort cloud, and the planets' orbits are essentially unchanged.

But on local scales it is a very different story. Since the initial fires of their births so long ago, the planets, their moons and even the Sun have seen significant changes. We will look at these in this, the third part of the book. Starting with the Sun and working our way outwards, we will investigate each of the elements of the Solar System in turn to see what they are like now – and how they have come to be that way. We shall see that the Sun is slightly larger and a bit more luminous today than it was when it was born. It will become evident why the Earth became the only place capable of supporting life, and why it no longer shows signs of its formation. We will discover a different Mars, a waterworld like Earth, and learn how it evolved into the cold and barren desert it is today. And there are other sights too: a larger Mercury than exists today; volcanoes that have changed planets' surfaces beyond recognition; and moons that have been shattered in cosmic impacts and later reformed into new and unusual forms. Having seen where the Solar System came from, it is time now to see how it has evolved.